Various medical substituting elements for hard tissues have been developed which are used as a substitute for part or the whole of the function of hard tissues such as a bone, ligament, and tendon impaired by an accident or the diseases such as rheumatism or osteoarthritis, during its treatment period or semipermanently.
For example, for medical treatment of a fracture, a fixation metal jig (bone fixation plate) is often used as the medical substituting element for hard tissues. This fixation metal jig is made of medical metal materials such as stainless steel, cobalt-chrome alloy, titanium, and titanium alloy. After a fracture is reduced and while the fractured bone is cured, the fixation metal jig is kept attached to a fracture with fixation screws, the jig traversing the broken position of the bone.
For medical treatment of a damaged ligament or a tendon, reconstruction and reinforcement are carried out using the tendon or fascia, or often using an artificial ligament or tendon. For the reconstruction and reinforcement by an artificial ligament or tendon, one or both ends of the artificial ligament or tendon are mounted with a reinforcing member such as a device with a boss which is fixed to the bone with fixation screws to thus fix the artificial ligament or tendon.
For medical treatments of a impaired joint, an artificial joint is often used as the medical substituting element for hard tissues. Artificial joints of various structures developed for specific applications include a hinge type, a hemi-hinge type, a rotating hinge type, a surface replacement type, a constraint surface replacement type, a stabilizer type, a meniscus bearing type, an unicompartmental replacement type, a tibia plateau type, an non-constraint type, and a hemi-constraint type.
Artificial joints are classified into two types, one having a constituent mounted on a bone with fixation screws and the other not having a constituent mounted on a bone with fixation screws. For example, an artificial knee joint of a conventional unicompartmental replacement type (a kind of surface replacement types) is of the type not having a constituent mounted on a bone with fixation screws. For example, an artificial knee joint of such an unicompartmental replacement type has a structure such as shown in FIG. 8.
FIG. 8(a) is a front view illustrating an artificial knee joint 30 of an uniconpartmental replacement type when in use, and FIG. 8(b) is a side view of the artificial knee joint 30 when in use. The artificial knee joint 30 (for right knee) shown in FIGS. 8(a) and 8(b) has a tibial component 33 and a femoral component 34, the tibial component 30 being constituted by a tibial tray 31 and a tibial bearing insert 32 fixedly pre-mounted on the upper surface (the surface face to the side of the femoral side when in use) of the tibial tray 31.
The tibial tray 31 has a plate unit 31a, and one long stem 31b and one short stem 31c formed on the bottom surface of the plate unit and slightly slanted, respectively made of medical metal materials including stainless steel such as SUS-316 type, cobalt-chrome-nickel alloy (COP), cobalt-chrome alloy (Vitallium), titanium, titanium alloy, and the like, medical ceramics materials including alumina, zirconia, hydroxyapatite, glass ceramics, and the like, or other medical materials.
The tibial bearing insert 32 is made of high density polyethylene (HDP), ultra-macromolecular polyethylene, or the like, and has a predetermined curve surface at its joint surface (the surface in contact with the femoral component 34 when in use). As described earlier, the tibial bearing insert 32 is fixedly pre-mounted on the upper surface of the tibial tray 31 and functions as a slide member for the femoral component 34.
The femoral component 34 has a plate unit 34a curved in a predetermined shape and two stems 34b formed on this plate unit 34a, respectively made of material including the above-described medical metal, ceramics, and other materials.
As described earlier, the artificial knee joint 30 is a unicompartmental replacement type artificial knee joint, and as shown in FIG. 9 substitutes part of the function of the knee joint. In mounting the artificial knee joint 30, the tibial component 33 is mounted on the proximal end of the tibia 40 at a predetermined position, and the femoral component 34 is mounted on the distal end of the femur 41 at a predetermined position.
In mounting the tibial component 33 on the tibia 40, for example, a predetermined area of the proximal end of the tibia 40 is ground-to form a flat surface, two holes for striking the two stems 31b and 31c of the tibial component 33 thereinto are formed in the flat surface area, thereafter the end of the longer stem 31b is first inserted into its hole, the tibial component 33 is rotated using the longer stem 31b as a fulcrum, and when the shorter stem 31c reaches its hole, the stems 31b and 31c are further struck into the holes. In mounting the tibial component 33, bone cement is also used if necessary.
On the other hand, in mounting the femoral component 34 on the femur 41, for example, after a guide hole, into which the stem 34b of the femoral component 34 is struck, is formed in the distal end of the femur 41 at a predetermined position, the distal end of the femur 41 is ground at a predetermined area and thereafter, the stem 34b is struck into the femur 41 along the guide hole. In mounting the femoral component 34, bone cement is also used generally. Reference numeral 42 shown in FIG. 9 represents a fibula.
Although a conventional medical substituting element for hard tissues of a type which is mounted on the hard tissues with fixation screws, is fixed to the hard tissues with fixation screws, a small degree of freedom is practically provided between the substituting element and fixation screws even after the mounting. The reason is that in order not to transmit an impact force applied to the hard tissues directly to the medical substituting element for hard tissues via the fixation screws, the fixation screws are given a capability of changing the direction of the impact force to provide a cushioning function. The other reason is that reverse rotation of the fixation screws is prevented in order for the medical substituting element for hard tissues not to be moved back outside. To this end, in a conventional medical substituting element for hard tissues, a through hole unit into which a fixation screw is entered is not formed with female threads, but only an engaging unit is provided which engages with the head of a fixation screw when it is threaded into the hard tissues to a certain degree, and restricts the further threading.
However, even if a medical substituting element for hard tissues is mounted with a degree of freedom between the element and fixation screws, vibrations applied to the hard tissues mounted with the medical substituting element transmit directly to the fixation screws, so that the fixation screws are spontaneously rotated and moved back outside, causing loosening of the medical substituting element, one of combined diseases.
Such loosening of a medical substituting element for hard tissues often occurs even for the type having a constituent mounted on a bone with stems struck into the bone, such as the conventional artificial knee joint of the unicompartmental replacement type shown in FIGS. 8 and 9. This occurrence frequency is conspicuous particularly for a constituent unable to use a long fixation stem, such as a tibial component.